Two examples of galaxy pairs in the COSMOS survey. The Hubble Space Telescope images show galaxies undergoing a close encounter (shown in gold). X-rays, as detected by Chandra, indicate which of the two galaxies hosts an AGN. In addition, diffuse X-ray emission from hot gas is present thus highlighting that such galaxy associations tend to reside in galaxy groups, an environment of rapid galaxy and black hole growth. Credit: Chandra X-ray Center

Most galaxies have supermassive black holes at their centers, as heavy as millions or even billions times that of our Sun. But how they grew to this tremendous weight is still a mystery. John Silverman from the Institute for the Physics and Mathematics of the Universe (IPMU) in Kashiwa, Japan, and the international COSMOS team have demonstrated, using the Chandra X-ray Observatory and the European Southern Observatory’s (ESO) Very Large Telescope (VLT), that interactions between galaxies are effective at triggering the growth of such supermassive black holes.

Supermassive black holes sit at the center of most galaxies that we observe today. How did they grow to millions or billions of times the mass of our Sun? Answers to this question have been elusive for some time, although important clues have been uncovered. For instance, supermassive black holes prefer to reside in the most massive galaxies, and the mass of a supermassive black hole is directly related to the mass of stars present in the central region (bulge) of its host galaxy. It is now thought that such massive galaxies grew, in part, by mergers and interactions between less massive galaxies. Such violent episodes in the evolution of galaxies have also been invoked to explain how matter is driven to their center that can then grow a supermassive black hole.

A simple test is to determine whether supermassive black holes are found in greater numbers in galaxies undergoing a merger compared to those in isolation. While this sounds easy enough, astronomers have struggled to effectively carry out this test for some time. This is because the glaring light of an actively growing supermassive black hole, seen observationally as either an active galactic nucleus (AGN) or the more luminous quasar, can outshine its entire host galaxy, making it difficult to determine whether it resides in a galaxy undergoing an interaction with another fainter galaxy. Such interactions should distort the shape of the galaxy.

The COSMOS team has carried out an alternative test that doesn’t require any knowledge of whether there are signs of distortions in optical images. Rather, galaxies are assumed to be in the process of interacting if they have a close neighbor. This requires accurate distance measurements of about 20,000 galaxies in the COSMOS field as provided by the COSMOS redshift survey with ESO’s VLT. Isolated galaxies provide the comparison sample to establish whether AGNs are more common in galaxies experiencing close encounters. X-ray observations with NASA’s Chandra observatory identify those galaxies that harbor AGNs because X-ray emission is a common property of growing supermassive black holes. Further, X-rays can penetrate obscuring material such as star-forming regions usually present in gas-rich galaxy mergers, thus providing a broad census of AGN activity.

It is reported that galaxies in close pairs are twice as likely to harbor AGNs as compared to galaxies in isolation. This result indicates that mergers, in an early stage, do lead to enhanced black hole growth. Given the moderate frequency of such interactions within the overall galaxy population, these events contribute about 20 percent to the overall mass buildup of the black hole population. Therefore, additional physical mechanisms are responsible for growing the majority of supermassive black holes. It is also likely that galaxies undergoing a final coalescence in a merger may play a role. These findings provide further evidence that galaxies and their supermassive black holes grow together over substantial periods of time.

Most galaxies have supermassive black holes at their centers, as heavy as millions or even billions times that of our Sun. But how they grew to this tremendous weight is still a mystery. John Silverman from the Institute for the Physics and Mathematics of the Universe (IPMU) in Kashiwa, Japan, and the international COSMOS team have demonstrated, using the Chandra X-ray Observatory and the European Southern Observatory’s (ESO) Very Large Telescope (VLT), that interactions between galaxies are effective at triggering the growth of such supermassive black holes.

Supermassive black holes sit at the center of most galaxies that we observe today. How did they grow to millions or billions of times the mass of our Sun? Answers to this question have been elusive for some time, although important clues have been uncovered. For instance, supermassive black holes prefer to reside in the most massive galaxies, and the mass of a supermassive black hole is directly related to the mass of stars present in the central region (bulge) of its host galaxy. It is now thought that such massive galaxies grew, in part, by mergers and interactions between less massive galaxies. Such violent episodes in the evolution of galaxies have also been invoked to explain how matter is driven to their center that can then grow a supermassive black hole.

A simple test is to determine whether supermassive black holes are found in greater numbers in galaxies undergoing a merger compared to those in isolation. While this sounds easy enough, astronomers have struggled to effectively carry out this test for some time. This is because the glaring light of an actively growing supermassive black hole, seen observationally as either an active galactic nucleus (AGN) or the more luminous quasar, can outshine its entire host galaxy, making it difficult to determine whether it resides in a galaxy undergoing an interaction with another fainter galaxy. Such interactions should distort the shape of the galaxy.

The COSMOS team has carried out an alternative test that doesn’t require any knowledge of whether there are signs of distortions in optical images. Rather, galaxies are assumed to be in the process of interacting if they have a close neighbor. This requires accurate distance measurements of about 20,000 galaxies in the COSMOS field as provided by the COSMOS redshift survey with ESO’s VLT. Isolated galaxies provide the comparison sample to establish whether AGNs are more common in galaxies experiencing close encounters. X-ray observations with NASA’s Chandra observatory identify those galaxies that harbor AGNs because X-ray emission is a common property of growing supermassive black holes. Further, X-rays can penetrate obscuring material such as star-forming regions usually present in gas-rich galaxy mergers, thus providing a broad census of AGN activity.

It is reported that galaxies in close pairs are twice as likely to harbor AGNs as compared to galaxies in isolation. This result indicates that mergers, in an early stage, do lead to enhanced black hole growth. Given the moderate frequency of such interactions within the overall galaxy population, these events contribute about 20 percent to the overall mass buildup of the black hole population. Therefore, additional physical mechanisms are responsible for growing the majority of supermassive black holes. It is also likely that galaxies undergoing a final coalescence in a merger may play a role. These findings provide further evidence that galaxies and their supermassive black holes grow together over substantial periods of time.